FIGS. 8(a) and 8(b) are a cross-sectional view and a top plan view, respectively, illustrating an InGaAs series semiconductor photodiode (hereinafter, referred to as a PD) as an example of a conventional photodetector. In the figures, reference numeral 1 designates a p side electrode comprising Ti/Au or the like. Reference numeral 2 designates a surface protection film comprising silicon nitride (SiN) or the like and having a thickness equal to 1/4 of wavelength .lambda. of light incident on the PD. Reference numeral 3 designates an undoped (hereinafter, referred to as i type) InP window layer having a thickness of about 2 .mu.m. Reference numeral 4 designates an i type InGaAs light absorption layer having a thickness of 2.about.3 .mu.m. Reference numeral 5 designates a p type doped region having a carrier concentration of 1.times.10.sup.19 .about.1.times.10.sup.20 cm.sup.-3, which is formed by diffusing Zn from the surface of the window layer 3 to reach into the light absorption layer 4, and its depth in the light absorption layer 4 is 0.1.about.0.3 .mu.m. Reference numeral 1a designates a bonding pad disposed on the surface protection film 2 in a region outside the Zn doped region 5. This bonding pad 1a is produced simultaneously with the p side electrode 1 and connected to the p side electrode 1. Reference numeral 6 designates a depletion layer. Reference numeral 7 designates an n type InP substrate containing a dopant impurity, such as silicon (Si) or sulfur (S), and having a carrier concentration of about 5.times.10.sup.18 cm.sup.-3. Reference numeral 8 designates an n side electrode comprising AuGe/Au. Reference numeral 9 designates incident light, for example, laser light having a wavelength .lambda. of 1.3 .mu.m, and reference numeral 10 designates reflected light. That is, a portion of the incident light 9, which has not been absorbed in the light absorption layer 4, is reflected at the n side electrode 8.
A description is given of the operation of the PD so constructed.
When light 9 having a wavelength .lambda. of 1.3 .mu.m is applied to the doped region 5 from the top of the PD, this incident light 9 travels through the surface protection film 2 and the InP window layer 3 and is absorbed in the InGaAs light absorption layer 4, thereby generating electrons and holes. The reason why the InP window layer 3 is transparent to the incident light 9 is because the incident light wavelength (.lambda.=1.3 .mu.m) is longer than the wavelength (.lambda.g=0.92 .mu.m) equivalent to the band gap energy of InP. At this time, in the light responsive region, i.e., a region of the i type InGaAs light absorption layer 4 opposite the doped region 5, a depletion layer 6 is produced by a pn junction that is formed between the p type doped region 5 and the n type substrate 7. That is, between the p type doped region 5 and the n type substrate 7, an electric field is applied in the InGaAs light absorption layer 4. Electrons and holes generated by light absorption in the depletion layer 6 in the light absorption layer 4 are pulled by the electric field and flow toward the n type InP substrate 7 and the p type doped region 5, respectively. This flow is called a photocurrent. The photocurrent is taken out as a signal current from the p side electrode 1 and the n side electrode 8. Further, a reverse bias of about -5V may be applied across the p side electrode 1 and the n side electrode 8 as desired.
A portion of the incident light 9, more specifically, several % of the incident light 9, is transmitted without being absorbed by the light absorption layer 4. When the light absorption layer 4 is sufficiently thick, for example, 5 .mu.m or more, it absorbs almost 100% of the incident light. However, when the light absorption layer 4 is thicker than 3 .mu.m, the light absorption layer 4 becomes slightly n type because a small amount of dopant impurity intrudes into the layer during the fabrication process although the light absorption layer 4 itself is intended to be undoped. Therefore, the light absorption layer 4 is not depleted at all, and generated electrons and holes remain in the light absorption layer 4 beneath the doped region 5. Accordingly, the thickness of the light absorption layer 4 is usually set at about 3 .mu.m or less and, as a result, a portion of the incident light 9 is not absorbed in the depletion layer 6 in the light absorption layer 4 but passes through the light absorption layer 4. The traveling light is reflected by the n side electrode 8, and the reflected light is input to an undepleted region (hereinafter referred to as a non-depletion region) in the light absorption layer 4 as well and absorbed in this region. Since the electric field generated in the depletion layer is not applied to the non-depletion region, electrons and holes generated by absorption of the reflected light 10 remain in this region. These electrons and holes reach the depletion layer 6 by diffusion and are taken out as a photocurrent. However, since it takes time for the electrons and holes to reach the depletion layer 6 after the light absorption, generation of this photocurrent in response to the incident light signal is delayed. When a photocurrent having such a delay is added to the photocurrent directly taken out of the depletion layer 6, distortion is generated in the response of the PD, i.e., the signal waveform is distorted. Although a PD is desired to have a sufficiently reduced response distortion, because the reflected light adversely affects the response of the PD as mentioned above, the response distortion increases to -80 dBc. As a result, it is impossible to realize a high-performance semiconductor photodetector.